Like trees, comets expel gas. But until now, researchers weren't exactly sure why.

Some researchers have argued the oxygen expelled by comets is as old as the solar system. Frozen inside the comet a few billion years ago, the molecular oxygen is finally thawed and sublimated from the comet's surface.

Others have argued oxygen atoms would have reacted with other chemicals during their lengthy lifespan.

In an effort to better understand the longevity of ancient oxygen, Konstantinos P. Giapis, a chemical engineer at the California Institute of Technology, decided to study the parameters of cometary chemical reactions.

"I started to take an interest in space and was looking for places where ions would be accelerated against surfaces," Giapis said in a news release. "After looking at measurements made on Rosetta's comet, in particular regarding the energies of the water molecules hitting the comet, it all clicked. What I've been studying for years is happening right here on this comet."

Giapis has spent several years studying ion-semiconductor collisions. The molecular collisions occurring on the surface of a melting comet are similar to those he's been studying in the lab for the last decade.

Water vapor escapes from the comet's surface as ice is melted by the sun's energy. The vapors get ionized by the sun's ultraviolet light. Solar winds blow the ions back toward the comet's surface where they collide with and pick up oxygen molecules trapped in the comet's surface. The oxygen atoms combine to form O2.

The new research, detailed in the journal Nature Communications, shows a comet doesn't need a frozen reserve of ancient oxygen to create O2 on its surface.

"We have shown experimentally that it is possible to form molecular oxygen dynamically on the surface of materials similar to those found on the comet," said postdoctoral researcher Yunxi Yao.

Giapis main focus in the lab is typically the creation of faster, more efficient computer chips.

"We had no idea when we built our laboratory setups that they would end up applying to the astrophysics of comets," said Giapis. "This original chemistry mechanism is based on the seldom-considered class of Eley-Rideal reactions, which occur when fast-moving molecules, water in this case, collide with surfaces and extract atoms residing there, forming new molecules. All necessary conditions for such reactions exist on comet 67P."

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